As a beverage can, a food can, and so on, there have conventionally been used containers (seamless cans) such as a drawn can (Patent Literature 1), a thinned redrawn can (Patent Literatures 2, 3), and a thinned redrawn-ironed can (for example, refer to Patent Literature 4). In the manufacture of these containers, a disk-shaped blank is first punched out from a plate material in which resin films are formed on one surface or both surfaces of a metal plate, and the blank is drawn, whereby a cylindrical primary drawn material (hereinafter, sometimes referred to as a “cup”) is formed. Next, the cup is redrawn and ironed.
In this method, mainly at the time of the redrawing and the ironing, the resin films at an edge portion of the blank are stretched, which is likely to cause the generation of hairy film pieces (hereinafter, sometimes referred to as “resin hairs”) (Patent Literature 6). Normally, the resin hairs have a length of about 10 mm or more. Then, the resin hairs sometimes exfoliate from the edge portion and adhere on an inner surface of the container to remain thereon. The resin hairs, though harmless to human bodies, are suspected as an unsanitary matter by consumers when mixed in the can, which can be a target of complaint. Therefore, when the generation of the resin hairs is confirmed during the manufacture of the container, all the products of the corresponding lot or the like are sometimes discarded. Thus, the generation of the resin hairs can have a great influence on a reduction in yield.
Under such circumstances, various arts aiming at the prevention of the generation of the resin hairs have been proposed (Patent Literatures 5 to 7). Patent Literature 5 describes an art relating to a method of punching out a blank from a plate material, and Patent Literatures 6 and 7 describe arts for removing part of resin films with a laser beam.
However, the art described in Patent Literature 5 is not capable of sufficiently preventing the generation of the resin hairs.
Further, according to the art described in Patent Literature 6 or 7, though the desired object is attained, its efficiency cannot be said to be high because an amount of the resin films that can be removed is not large, considering an energy amount required for the irradiation of the laser beam.